1. Field of the Invention
This invention relates to a magnetic tape cassette in which an opening through which a tape loading mechanism of a recording and reproducing system is inserted into the cassette body is formed in the bottom of the casing and a slider is provided to be slid back and forth to open and close the opening, and more particularly to the structure of the slider.
2. Description of the Related Art
In such a magnetic tape cassette, a pair of hubs around which a magnetic tape is wound are contained in a casing body spaced from each other in a direction transverse to the direction of loading of the magnetic tape cassette into a recording and reproducing system, and an opening which gives a tape loading mechanism of the recording and reproducing system access to the tape is formed in the bottom of the casing body at the front end portion thereof. In order to prevent entrance of dust and/or other foreign matters into the casing body through the opening, a slider is provided to be slid back and forth along the bottom of the casing body in the direction of loading to open and close the opening.
Mechanisms for opening the slider in response to loading the magnetic tape cassette into the recording and reproducing system and closing the slider in response to unloading the cassette from the system are disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 60(1985)-129985, 60(1985)-136078, 60(1985)-136079 and 60(1985)-175271.
When the slider is in contact with the bottom surface of the casing body over the entire area thereof, slight manufacturing errors, internal strains or thermal deformation can result in an excessive sliding resistance, which can cause failure in action of the slider. In order to overcome this problem, there has been developed a technique in which ribs are formed on the surface of the slider to extend in the direction of loading so that the slider is in contact with the bottom surface of the casing body only at the ribs. See, for instance, U.S. Pat. No. 4,660,784. Further in the known technique, a locking mechanism for locking the slider in its closing position and opening position is provided and a channel for guiding a lock release member which releases the lock is formed on the bottom surface of the slider.
FIG. 8 shows a slider 5 in the tape cassette of the prior art. As shown in FIG. 8, the slider 5 is provided on its upper surface with a pair of protrusions 220 which are in a sliding contact with the lower surface of a lower casing half 3 (FIG. 1) of a casing body 1 (to be described later). The protrusions 220 extend over the entire length of the slider 5 from the front end to the rear end of the slider 5. Each protrusion 220 is provided with through holes 15 and 16 which are selectively brought into engagement with a resilient locking piece on the casing body 1 in the closing position and the opening position of the slider 5. A guide channel 17 for guiding a lock release member which disengages the locking piece from the through holes 15 and 16 to release the lock is formed on the lower side of each protrusion 220. The other elements of the slider 5 shown in FIG. 8 are given the same reference numerals as in FIG. 1 which will be described later, and will not be described here.
However the slider of the prior art is disadvantageous in that due to the guide channel formed in the protrusion, a thick wall portion and a thin wall. portion are formed side by side, which is apt to cause warpage in a molded slider and deteriorate the dimensional accuracy, thereby causing various drawbacks.
That is, the part of the protrusion where the guide channel is formed is smaller in wall thickness than the parts on opposite sides of the channel and accordingly resin cannot smoothly flow into the cavity portion of the mold corresponding to the part, which often results in warpage, local deformation and/or surface defects such as failure in transfer of a grain pattern, weldline, irregularity in surface appearance, sink marks and the like.
Further when the protrusions extend over the entire length of the slider, a foreign matter caught between the protrusion and the casing body cannot be discharged since the direction and length of the sliding action of the slider are constant and keeps obstructing sliding action of the slider for a long time.